In advanced information community, communication technique, particularly, wireless communication technique is required to be developed. One of the wireless communication technique is that a base station (herein, called as a “transmitting station”) provided in a cell (called as a “local area cell”) having a radius of several hundred meter and plural mobile stations (called as a “receiving station”) moving in the local area cell wirelessly communicate one another at the same time. In such a wireless communication, plural channels which do not interfere with one another must be installed. Therefore, as a multiple access system, Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) or the like are suggested. This invention belongs to'the Code Division Multiple Access (CDMA) in these multiple access systems.
The CDMA is also called “Spread Spectrum Communication (SSC) system because its frequency spectrum is spread by modulating baseband data with a high speed digital code. The SS-CDMA has excellent characteristics of fading-resistance, multipass-resistance and interference-resistance, and has distribution exchange function and position-determining function.
In this invention, a “down link” means a two-way transmission from a base station localized almost at the center of a cell to plural mobile stations in the cell. Hereinafter, a “transmitting station” means the base station, and a “receiving station” means the mobile station. In the down link of such a SS-CDMA (communication from the transmitting station to the receiving station), baseband data are multiplied by a carrier having a given center frequency in the transmitting station to generate a primary modulation signal, by which a spread code (pseudo-noise (PN) code) is multiplied to transmit a secondary modulation signal having a spread spectrum. In the receiving station, the same spread code and the same carrier as in the transmitting station are generated, and multiplied by the received signal to demodulate the original baseband data.
As a spread code in such a SS-CDMA communication system, an orthogonal code is employed to identify many channels and, for example, an orthogonal m-sequence code, an orthogonal Walsh code, an orthogonal Gold code or the like may be exemplified. In employing such an orthogonal code, the timing of the generation of a given orthogonal code in a receiving station is required to be synchronized with that in a transmitting station. This code synchronization is called as a “chip synchronization”. Conventionally, in order to attain the chip synchronization, a digital sliding correlation apparatus or a digital matched filter is suggested.
The digital sliding correlation apparatus circulates the orthogonal code at a higher speed than a received signal, and attains the chip synchronization by a detector having a DLL (Delay Lock Loop). However, it is disadvantage that the digital sliding correlation apparatus becomes unstable in its operation due to the balance of the correlation apparatus and requires long time in synchronization acquisition because it has to circulate the code by maximum one period.
Moreover, the digital matched filter detects the correlation peak between a known orthogonal code and a received signal by correlation-integrating both of them using a shift resistor, and thereby, attains synchronization acquisition. Although the digital matched filter can attain the synchronization at a higher speed than the above digital sliding correlation apparatus, it may make the timing of the correlation peak ambiguous. Moreover, in the digital matched filter, as the number of the chips in the orthogonal code per period increases, the bits in the shift resistor increase, which results in the deterioration of the economical efficiency thereof.
The digital matched filter using a silicon integrated circuit technique generally operates in a baseband frequency, so that it can not operate when it includes a carrier frequency. Therefore, the received signal has to be input into the digital matched filter after synchronous detection, etc. Generally, a secondary modulated signal by a PN code has a spread spectrum, and has much difficulty in the synchronous detection due to its small C/N ratio. In principle, if a silicon integrated circuit using a microfabrication technique of 0.2-0.13 μm which is under development is employed, the digital matched filter which operates at around 100 Hz can be obtained. However, it is difficult to use the digital matched filter for the receiver of a mobile machine requiring a low electric power consumption because the silicon integrated circuit has a large-scale circuit which means that an electric power consumption is larger than at least 1 watt.
Moreover, it is disadvantage that the digital sliding correlation apparatus and the digital matched filter have large electric power consumption at stand-by state.
To solve the above problems, the present inventors suggest the following code division multiple access communication system. First, in a transmitter, a code division multiple signal which is composed of a data division obtained by multiplying an orthogonal code by a baseband and a preamble division to synchronize the orthogonal code in a receiver is transmitted. Second, in the receiver, the correlation peak is detected from the synchronized code sequence in the preamble division by an surface acoustic wave matched filter. At last, the orthogonal code is generated based on the detection timing, and the received baseband data in the data division is demodulated by the orthogonal code. Such a code division multiple access communication system is described in for example, Kokai Publication 9-261121(JP A 09-261121). Hereinafter, the surface acoustic wave matched filter is called as a “SAW MF”. The “SAW MF” is abbreviated from the wording “Surface Acoustic Wave Matched Filter”.
The above-mentioned code division multiple access communication system can synchronize the orthogonal code at a high speed. Moreover, the surface acoustic wave matched filter is a passive device, and has small electric power consumption, so that it can essentially provide a receiving station having a small stand-by electric power. Furthermore, since the surface acoustic wave matched filter can correlate the code sequence of surface acoustic wave matched filter with a code division multiple signal including a carrier, it can correlate in a GHz band or RF band if it can made of a suitable material for such bands. Therefore, since a received signal in a RF band is directly input and correlated in the surface acoustic wave matched filter, a pretreatment such as a down converting is not advantageously required. As described later, a SAW MF having an “aluminum nitride/sapphire” structure including an aluminum nitride film is suitable for the SAW MF which can operate in the GHz band.
In the conventional code division multiple access communication system described in the above Kokai Publication 9-261121(JP A 09-261121), the preamble division is composed of a packet division for synchronization having a Barker code of 11 chips as a code sequence for synchronization and a dummy division of 5 chips next to packet division for the synchronization, and the data division is composed of n-sequential symbols of 1024 chips demodulated by the orthogonal code. As mentioned above, in the conventional code division multiple access communication system, there is the preamble division in the receiver in order to generate the synchronized orthogonal code with the chips in the data division of the received code division multiple signal. However, the code division multiple signal has only one code sequence for synchronization at the front of one packet, so that the packet can not be received entirely if the code sequence for synchronization is not detected. Since there are large influences resulted from various noises, multipass and cross talk between the adjacent cells in wireless communication, the correlation peak of the code sequence for synchronization can not be detected in good condition if the preamble division has only one code sequence for synchronization.
Moreover, in the conventional code division multiple access communication system, the receiver generates the carrier which is synchronized with the carrier of the code division multiple access signal made from correlation peaks, i.e. output signal from the surface acoustic wave matched filter, and combines the generated carrier with the orthogonal code generated as mentioned above, and demodulates the baseband data by multiplying the combined signal by the received signal. The synchronized carrier with received carrier in phase and frequency can be reproduced, in the period when the correlation peak of the surface acoustic wave matched filter appears. However, the communication circuit is required to be devised to reproduce the carriers in the short period when the correlation peak appears. Therefore, the circuit structure using a simple method is desired.
It is a first object of the present invention to provide a code division multiple access communication system which can attain the chip synchronization in the code sequence for synchronization precisely at a high speed even under a bad communication environment, and thus, can remove the disadvantage of being incapable of receiving the packet entirely.
It is a second object of the present invention to provide a code division multiple access communication system which can demodulate the baseband data precisely from the correlation peak of the surface acoustic wave matched filter without the carrier synchronized to the received signal, in addition to realizing the first object.
This invention relates to a code division multiple access communication system in which in a transmitter, a code division multiple access signal, composed of a data division obtained by multiplying a baseband data and an orthogonal code and a preamble division including synchronization code sequences to attain the chip synchronization of the orthogonal code in a receiver, is modulated with a carrier having a given center frequency and transmitted, and in the receiver, a correlation peak is detected, from among the synchronization code sequences in the preamble division by a surface acoustic wave matched filter and the baseband data in the data division is demodulated by the orthogonal code generated on the detection timing, wherein the preamble division has plural synchronization code sequences, and the surface acoustic wave matched filter detects the correlation peak of at least one from among the plural synchronization code sequences and generates the orthogonal code on the detection timing of the correlation peak.
According to the code division multiple access communication system of the present invention, the orthogonal code with the chip synchronization can be generated if at least one of the plural synchronization code sequence in the preamble division of the transmitted signal can be detected by the surface acoustic wave matched filter, which prevents the dropout of the whole packet.
As mentioned above, even if the preamble division has the plural synchronization code sequences, the surface acoustic wave matched filter can not always detect the correlation peaks of all the synchronization code sequences. Thus, even if all the same synchronization code sequences are employed, the timing of the generation of the orthogonal code can not be estimated from the detected correlation peaks. That is, if the preamble division has 10 synchronization code sequences for example, the surface acoustic wave matched filter detect the correlation peaks from all the 10 synchronization code sequences at every detection thereof in an idealistic condition. If only 9 synchronization code sequences are detected due to the deterioration of the communication path, generally, the starting time of the first symbol in the data division can not be estimated from the correlation peaks of these 9 synchronization code sequences. To solve such a problem, it is preferable to realize the synchronization code sequence corresponding to the correlation peak by changing the construction of the plural synchronization code sequences, but it results in the complication of the construction of the transmitter or the receiver.
In this invention, for ironing out this problem, it is preferable that in a code division multiple access communication system in which in a transmitter, a code division multiple access signal, composed of a data division obtained by multiplying a baseband data and an orthogonal code and a preamble division including synchronization code sequences to attain the chip synchronization of the orthogonal code in a receiver, is modulated with a carrier having a given center frequency and transmitted, and in the receiver, a correlation peak is detected from among the synchronization code sequences in the preamble division by a surface acoustic wave matched filter and the baseband data in the data division is demodulated by the orthogonal code generated on the detection timing, the period of the synchronization burst in the preamble division (Tburst) which is composed of a synchronization packet division having at least one synchronization code sequence and a dummy division next to the packet division, is set equally to the integral multiples of the period of one symbol in the data division (Tsymbol). In particular, it is most preferable that Tburst is equal to Tsymbol in the data division.
According to the preferred code division multiple access communication system, in the case that the preamble division has 10 synchronization code sequences having the same structure, for example, if at least one correlation peak from among the synchronization code sequences is detected, the orthogonal code can be generated, in accordance with the start timing of the first symbol in the data division.
Moreover, in this case, the orthogonal code can be generated on the timing of the first detection of the correlation peak from the synchronization code sequences in the preamble division, and the operation of the orthogonal code generating circuit can be reset at every detection of the correlation peaks as described later in an embodiment. Since the latter case can generate the orthogonal code on the timing much near the start timing of the data division, it may attain the chip synchronization more precisely.
Moreover, the present invention to realize the second object as mentioned above relates to a code division multiple access communication system in which in a transmitter, a code division multiple access signal, composed of a data division obtained by multiplying a baseband data and an orthogonal code and a preamble division including synchronization code sequences to attain the chip synchronization of the orthogonal code in a receiver, is modulated with a carrier having a given center frequency and transmitted, and in the receiver, a correlation peak is detected from among the synchronization code sequences in the preamble division by a surface acoustic wave matched filter and the baseband data in the data division is demodulated by the orthogonal code generated on the detection timing, wherein in the receiver, the orthogonal code which is generated on the detection timing of the correlation peak in the surface acoustic wave matched filter is multiplied by the received code division multiple signal to generate a narrow-band modulation signal, and the generated narrow-band modulation signal is demodulated by a carrier generated from a local oscillator provided in the receiver to reproduce the original baseband data.
In a preferred embodiment of the code division multiple access communication system according to the present invention, the carrier having the frequency equal to the center frequency of the carrier generated in the transmitter is generated, and is multiplied by the narrow-band modulation signal to demodulate the baseband data.
Moreover, the carrier having a different frequency from the center frequency of the carrier generated in the transmitter is generated, and the narrow-band modulation signal having their differential frequency is generated by multiplying the generated carrier and the narrow-band modulation signal. Then, the narrow-band modulation signal having their differential frequency is demodulated and thereby, the baseband data is demodulated. This system is called a “Heterodyne system”.
In both cases, normal demodulation system may be employed in reproducing the baseband data by demodulating the narrow-band modulation signal.
Furthermore, in the present invention, it is ascertained that when the repeated number Nburst in the plural bursts constituting the preamble division is set to 5-15, particularly 6-12, the probability of the dropout of the whole packet due to not attaining the chip synchronization is much less than that of the conventional code division multiple communication access system.
In this invention, it is preferable that the chip rate of the synchronization code sequence in the preamble division is higher than the chip rate of the orthogonal code in the data division, particularly, integral times of not less than two as high as the chip rate of the orthogonal code in the data division. In this case, since the correlation peak output from the surface acoustic wave-matched filter is sharper in terms of time, the generation timing of the orthogonal code can be defined more precisely.
As mentioned above, as the synchronization code sequence in the preamble division, the Barker code of 11 chips, the M-sequential code or the like may be employed, and as the orthogonal code in the data division, the orthogonal m-sequential code, the orthogonal Walsh code, the orthogonal Gold code or the like may be employed.